In general, esters and ethers are organic molecules that contain oxygen in the following structural configuration: ##STR1## where R.sup.1, R.sup.2, R.sup.3 and R.sup.4 represent organic groups. Esters and ethers may exist in polymeric form, as shown by the following example formulas: ##STR2## where n represents a large integer, and the material within parentheses represents a "repeating unit" which may contain one or more ester linkages.
It is possible to fluorinate ester to ether compounds. In such a reaction, fluorine atoms are substituted for hydrogen atoms, as exemplified by the following equation: R1 ? ##STR3## If the fluorine substitution reaction proceeds to completion, i.e., if all of the hydrogen atoms are replaced by fluorine atoms, the resulting compound is designated as "perfluorinated", as exemplified by the following equation: ##STR4## A perfluorinated organic group is referred to herein by the symbol R.sub.f, while a partially fluorinated organic group is referred to by the symbol "R.sub.h/f."
The perfluorination concept also applies to polymers. For example, a "perfluoropolyether" would indicate a polymeric ether molecule wherein all (or substantially all, as described below) of the hydrogen atoms have been replaced by fluorine atoms.
An organic substance may be fluorinated by any of several techniques which are known to those skilled in the art. Such techniques may be divided into two categories, which are commonly referred to as "direct fluorination" and "indirect fluorination".
Direct fluorination implies that a substance is contacted with fluorine that is in the elemental form, as fluorine gas (F.sub.2). This type of reaction is highly exothermic, and can lead to adverse effects such as breakage of carbon-carbon bonds. Therefore, direct fluorination is often carried out at low temperatures using fluorine gas which is diluted during the initial contact with inert gas such as helium or nitrogen. See, e.g., U.S. Pat. No. 4,281,119, (Lagow et al., 1981). The concentration of fluorine may be increased gradually or stepwise until pure fluorine gas surrounds the substance. The temperature and pressure of the gas may also be increased during the fluorination process.
Indirect fluorination indicates that the substance to be fluorinated is contacted with a compound that contains fluorine, such as sulfur tetrafluoride (SF.sub.4), sulfur hexafluoride (SF.sub.6), or molybdenum hexafluoride (MoF.sub.6). When heated or otherwise manipulated, such compounds may be induced to release fluorine atoms. The released fluorine atoms react with the contacted substance to produce a desired fluorinated or perfluorinated substance. Several monomeric perfluoroether compounds, and methods for producing such compounds, are known. See, e.g., G. E. Gerhardt et al., J. Polymer Science: Polymer Chemistry Ed. 18: 157-168 (1979); U.S. Pat. No. 3,985,810 (von Halasz et al., 1976). Such compounds suffer from the following shortcomings:
1. There are no commonly available perfluorinated polyethers with more than two or three carbon atoms between adjacent oxygen atoms, i.e., ##STR5## is not commonly available. 2. There are no commonly available perfluorinated polyether compounds with alternating units, e.g., EQU --(R.sub.f.sup.1 --O--R.sub.f.sup.2 --O--)-- PA0 3. Most perfluoropolyethers are created by fluorinating perfluoro vinyl epoxide monomers or perfluoro-epoxide monomers, e.g., ##STR6## However, it is difficult to synthesize precursor perfluoroepoxides, especially epoxides that contain bulky fluorocarbon groups. It is also difficult or impossible to polymerize perfluoro epoxides containing bulky perfluoro groups (R.sub.f.sup.1 -R.sub.f.sup.4). Either no polymerization occurs or very low molecular weight products are obtained. PA0 1. The polyethers were not perfluorinated, and could not be perfluorinated by known techniques without creating substantial quantities of undesired by products. PA0 2. The product of the SF.sub.4 reaction has a certain amount reactive acid end groups. The only use that was cited for this compound requires crosslinking and curing steps of convert the compound into an elastomer.
Several polymeric perfluoroethers are also know. Such compounds are believed to be highly stable; for example, the only reported reaction of saturated perfluoropolyethers known to the Applicants is chain cleavage at the ether linkage by aluminum chloride at elevated temperatures and high pressure. See, G. V. D. Tiers, J. Amer. Chem. Soc. 77: 4837 (1955). Saturated perfluoropolyethers also tend to have relatively broad liquid ranges, with viscosities and surface properties which make them attractive for numerous applications such as solvents, hydraulic fluids, heat-transfer fluids, lubricants, greases, sealants, elastomers and plastics.
It has been shown that various types of polymeric perfluoroethers can be produced by direct fluorination of hydrocarbon polyethers. See, e.g., G. E. Gerhardt, et al., J. Org. Chem. 43: 4505 (1978); G. E. Gerhardt, et al., J.C.S. Perkin I 1321 (1981). However, the generality of this method is limited by the fact that relatively few hydrocarbon polyether compounds are commercially available. In order to produce a wide range of perfluoropolyether compounds, numerous polymeric precursors with a variety of repeating units would need to be available.
It is possible to create fluoroethers by fluorinating ester compounds. A known reaction for accomplishing this result utilizes SF.sub.4 as a fluoridating agent [W. R. Hasek, et al., J. Amer. Chem. Soc. 82: 543 (1960); W. C. Smith, Angew. Chem. Int. Ed. 1: 467 (1962); W. A. Sheppard, J. Org. Chem. 29: 1-11 (1964)] according to the following reaction: ##STR7##
This reaction has been utilized to convert various highly fluorinated monoesters into highly fluorinated monoethers. R. J. De Pasquale, J. Org. Chem. 38: 3025 (1973).
The SF.sub.4 reaction has also been utilized to create certain forms of polyethers as well. U.S. Pat. No. 4,201,876 (Griffin, 1980); U.S. Pat. No. 4,238,602 (Griffin, 1980). However, the polyether compounds created by SF.sub.4 suffered from several limitations:
The background art did not provide a general method for creating stable perfluorinated polyethers with a sufficient variety of repeating units.